1. Field of the Invention
The present invention relates to a method for fabricating a contact plug in a highly integrated semiconductor device, and more particularly to a method for fabricating a tungsten contact plug buried in a via contact hole for a connection among metal layers of a multilayer structure.
2. Description of the Prior Art
Contact plugs made of aluminum alloy are well-known. In semiconductor devices with a higher integrity, however, such contact plugs encounter a variety of problems because the coverage of the aluminum alloy are poorer. For solving such problems, there has been proposed a method for fabricating a contact plug by a selective deposition of a tungsten layer. In accordance with this method, a tungsten layer is selectively deposited over an aluminum alloy layer at a high temperature of no less than 400.degree. C. by use of a reacting gas consisting of WF.sub.6, SiH.sub.4 and H.sub.2. When tungsten (W) is grown over aluminum alloy, the aluminum and the WF.sub.6 gas react with each other at an interface between Al and W, thereby producing AlF.sub.3 adversely affecting the contact resistance, as expressed by the following equation (1): EQU Wf.sub.6 +2Al.fwdarw.W+2AlF.sub.3 ( 1)
On the other hand, a reacting gas used upon forming a via contact hole includes a fluorine (F)-based chemical material such as CHF.sub.3, CF.sub.3 or O.sub.2. By such chemical material, an F-based polymer may be formed on the Al surface. When Al alloy is deposited, the F-based polymer forms voids which make it difficult to achieve a subsequent W deposition.
Where a tungsten contact plug is formed in a structure including a first metal layer made of aluminum alloy, a second metal layer made of both an aluminum alloy and an insulating film comprised of a spin-on-glass (SOG) film, gas is generated from the SOG film at the interface between W and Al during the W deposition for the contact plug. The gas may penetrate the interface between w and Al and adversely affect the contact resistance. Such gas-generating phenomenon is sensitive to temperature and becomes more active at a higher temperature of, for example, not less than 250.degree. C.